A video signal output circuit which outputs a video signal to a video signal output terminal and the like of an electronic device generally uses a sync-tip clamp circuit for fixing the level (sync-tip level) of the bottom (sync-tip) of a synchronous signal at a constant voltage VSC, as in the waveform diagram shown in FIG. 3 (e.g., Japanese Patent Laying-Open No. 06-225181 (Patent Document 1)). Furthermore, in recent years, a video signal output circuit has also been developed which incorporates a low-pass filter and is capable of directly inputting a video signal output from a D/A converter to a video signal output terminal of an electronic device.
FIG. 4 is a circuit configuration diagram of a conventional video signal output circuit.
Referring to FIG. 4, in a conventional video signal output circuit 101, a coupling capacitor 102 eliminates a DC component of the video signal input to an input terminal IN, and outputs an AC component to a sync-tip clamp circuit 103 at the subsequent stage. Sync-tip clamp circuit 103 fixes the sync-tip level at a constant voltage and outputs the video signal to a low-pass filter 104 at the subsequent stage. Low-pass filter 104 eliminates unnecessary sampling noise and outputs the video signal to an output driver 106 such as a 75Ω driver at the subsequent stage. Output driver 106 outputs the video signal to an output terminal OUT with a low output impedance.
Low-pass filter 104 of video signal output circuit 101 is configured to use a differential amplifier 145. This prevents the sync-tip level of the video signal output from low-pass filter 104 from deviating from the sync-tip level of the video signal output from sync-tip clamp circuit 103. Therefore, low-pass filter 104 can almost maintain the sync-tip level of the video signal obtained by sync-tip clamp circuit 103. Accordingly, the input level of output driver 106 can be stabilized and the maximum output level (a maximum output peak-to-peak voltage) of the video signal to be output can be raised.
FIG. 5 is a detailed circuit diagram of low-pass filter 104. Differential amplifier 145 includes a pair of input transistors 201, 202, a pair of load transistors 203, 204, an input stage constant current source 205, an output transistor 206 and an output stage constant current source 207. The pair of input transistors 201, 202 is generally large in size so as to obtain a good pair characteristic.
Patent Document 1: Japanese Patent Laying-Open No. 06-225181